It is commonplace in semiconductor device manufacture to utilize layers of materials such as silicon nitride and silicon oxide. These layers are patterned to cover selected regions of the semiconductor wafer or other substrate. The patterned layers are used as etch masks during wafer processing or as insulating dielectrics which electrically isolate different conductors or device regions, or both. An example of the use of silicon nitride and silicon oxide layers for fabrication of semiconductor devices and integrated circuits is described in U.S. Pat. No. 4,199,380 to M. G. Farrell et al., which is incorporated herein by reference.
During wafer processing, it is important to be able to etch materials selectively, that is, to be able to etch one material at a much faster rate than another material when both materials are simultaneously exposed to the same etchant. Etch selectivity permits the slower etching material to be used as a mask for the faster etching material. Silicon nitride and silicon oxide are known to form a pair of materials exhibiting etch selectivity. Well-known reagent systems exist for carrying out selective etching of silicon nitride and silicon oxide by wet chemistry.
More recently, there has arisen great interest in dry etching, that is, chemical etching systems which do not use wet chemistry. It has been found that many of the materials used in semiconductor manufacture can be etched by exposure to gaseous plasmas containing the proper active species. These techniques are generally referred to as reactive ion etching or plasma etching.
It is well-known that silicon based materials can be etched by plasmas formed from certain halogen compounds, carbon-halogen compounds, and mixtures thereof. For example, it is reported in U.S. Defensive Publication No. T101,302 that silicon can be preferentially etched in a plasma formed from CF.sub.4 +Cl.sub.2. It is reported in U.S. Pat. No. 4,414,057 that silicon can be etched using a plasma formed in a mixture of C.sub.2 F.sub.6 +Cl.sub.2. It is reported in U.S. Pat. No. 4,406,733 that PF.sub.5 and PCl.sub.3 are useful for etching silicon, in U.S. Pat. No. 4,380,489 that SF.sub.6 is useful for etching silicon, and in U.S. Pat. No. 4,310,380 that ClF.sub.3 or NF.sub.3 or BrF.sub.3 or IF.sub.3 or ClF.sub.3 +Cl.sub.2 are also useful for etching silicon. These patents and defensive publication are incorporated herein by reference.
It is further known that silicon oxide and silicon nitride can be etched in gaseous plasmas. For example, U.S. Pat. No. 4,352,724 describes plasma etching of silicon nitride and silicon oxide using mixtures of various carbon-halogens and oxygen, in particular for etching silicon oxide C.sub.2 F.sub.5 Cl+CF.sub.4, or CF.sub.4 +O.sub.2, or CF.sub.4 are useful, and for etching silicon nitride, C.sub.2 F.sub.5 Cl+CF.sub.4 +O.sub.2, or CF.sub.4, or CCl.sub.4, or PCl.sub.3, or BCl.sub.3, or SiCl.sub.4, or other chloride gases with O.sub.2 or CF.sub.4 are useful. It is reported in U.S. Pat. No. 4,374,698 that silicon nitride and silicon oxide can be selectively plasma etched using mixtures of CF.sub.4 +CF.sub.2 Cl.sub.2 +O.sub.2, or CF.sub.4 +CF.sub.3 Br+O.sub.2, or CF.sub.4 +CF.sub.3 Br+NO, and that ratios of nitride etch rate to oxide etch rate of 5:1 to 12:1 can be obtained, with nitride etch rates ranging from about 2 to 25 millimicrons per minute. These patents are incorporated herein by reference.
Nitride to oxide etch rate ratios above about 2:1 are sufficient to permit selective etching of nitride layers on top of oxide layers, which is a frequently encountered situation. However, larger etch rate ratios are desirable. A deficiency of the prior art procedures and gas compositions for plasma etching of nitride and oxide layers is that they employ carbon-halogen compounds. It has been found that carbon-halogen compounds are a source of contamination within the plasma chamber, particularly when hydrogen bearing compounds such as resists are present on the substrate or in the chamber during plasma etching, as is common. Carbon dissociated from the carbon-halogen compounds forms non-volatile complexes which deposit on the walls of the plasma reactor and on the semiconductor substrates. This produces undesirable contamination.
Thus, a need continues to exist for plasma etching methods and materials which avoid the contamination problems associated with prior art methods and gases and which provide adequate selectivity and etch rates.
Accordingly, it is an object of the present invention to provide an improved method and gas mixture for selective plasma etching of nitride compounds and oxide compounds which avoids carbon or hydro-carbon related contamination products.
It is a further object of the present invention to provide an improved plasma etching method and plasma gas which avoids carbon related contamination while achieving selective etching of nitride to oxide.
It is an additional object of the present invention to provide an improved plasma etching method and plasma gas which avoids carbon related contamination, which achieves absolute nitride etch rates as well as selective nitride to oxide etch ratios in the same range or higher than the prior art.
It is a further object of the present invention to provide an improved plasma etching method and plasma gas which uses readily available and easily purified source gases which are relatively inexpensive and easy to handle during wafer manufacturing.